1. Field of the Invention
The present invention relates to a nonlinear optical compound which can be used in various optical devices such as a high speed optical switch and an optical memory.
2. Background of the Invention
As a photonics material for optical devices, a compound or a material which has a large nonlinear optical effect and high speed response is desired, and a wide variety of compounds or materials are studied.
As such compound, an organic compound having a .pi. electron conjugated system is considered to be better than a crystalline inorganic material in which lattice vibration contributes to the nonlinear optical effect. To design such organic compound, it is generally and strongly proposed to introduce at least one strong donor type (electron donative) group and at least one acceptor type (electron attractive) group in a molecular having the .pi. electron system.
Alternatively, to introduce an organic compound which exhibits strong nonlinear optical characteristics in a polymer, there are proposed a method comprising crystallizing a compound each molecule of which has a large dipole moment while controlling molecular orientation by applying an electric field so as to exhibit the nonlinear optical characteristics more effectively (Japanese Patent Kokai Publication Nos. 274122/1989 and 188835/1989), a method comprising introducing a nonlinear optical material in side chains of a polymer, and a method comprising crosslinking such compound with polymer chains through a chemical reaction between the compound molecules and the polymer chains.
However, in case of the organic compound having the .pi. electron conjugated system in which the strong electron donative and electron attractive groups are introduced, its crystallization is difficult, or if it is crystallized, the nonlinear optical characteristics are not realized because, due to a very large dipole moment, the compound is crystallized with compensating the dipole moments of the molecules each other.
When a .pi. electron conjugate length is elongated, an absorption edge of the material shifts to a long wavelength side, so that the material absorbs a fundamental wave and higher harmonics, whereby an output is decreased.